Quote:
Originally Posted by LubeMaster77
Typically, a fractured piece will have a dark area which is an indication of slow crack growth, and a bright area which is an indication of sudden fracture. Fatigue begins with the crystalline structure experiencing dislocation. Eventually small micro-fissures develop which begin to compromise the physical integrity of the part, tool or material. Many materials will not normally recover if fatigue is relieved.
So, answer me this students-o-failure, why would the color be dark on the inside and not on the outside? How do you think the stress developed?
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The highest stresses on a shaft in torsion are at the surface, and the crack would start there if there was no keyway. Except, there is a keyway and it has sharp edges where it was machined into the shaft. The the sharp edges of the root of the keyway caused localized stresses high enough to cause a fatigue crack to initiate. If it had grown slowly the first part of the crack would have darkened by oxidation, but this one grew fast enough for the fretting (rubbing of the sides of the broken crack together) to keep it polished. The dark area on the inside of the shaft is where the remaining material was stressed beyond the point of ultimate failure and came apart. It appears dark because it is a rough surface. This might have been avoided if the corners in the root of the keyway had been radiused.